Collaborative Research: A global examination of the subduction zone flow field from seismic anisotropy

当一个构造板块重新陷入地球时会发生什么？在俯冲带的内部？令人惊讶的是，这个简单的问题仍然是地球科学中未解决的基本问题之一。下行板块（通常称为板块）周围的流动模式通常使用地震各向异性进行研究，地震各向异性是地幔的一种属性，可以通过记录地震波来检测。特别地，剪切波分裂的测量可以用于表征俯冲带中的地震各向异性。从这些测量中越来越清楚地看出，平板上方二维角流和平板下方夹带流的最简单模型可能是不正确的。然而，尚未就替代模式达成共识。在这个项目中，我们将进行剪切波分裂观测的全球调查，目的是了解是什么控制着俯冲带地区的地幔流场。要做到这一点，我们将确定参数，描述俯冲带动力学，似乎施加一阶控制剪切波分裂。初步工作已经确定了与沟槽迁移速度的大小相关的板块上方和下方的各向异性的系统变化。这导致了假设，3-D流占主导地位的板下，并与2-D的地幔楔角流相互作用。除了在俯冲带的地震各向异性的系统评价，我们将建立实验室和数值模型的地幔流以上和以下的板，以确定诊断功能的流场剪切波分裂测量和探讨我们的模型的地幔动力学的影响。该项目构成了一个跨学科的努力，了解和表征地幔流场的特点，伴随着俯冲使用地震各向异性和地球动力学建模的观察。随着剪切波分裂作为地幔流制图工具的日益普及，俯冲带的数据越来越丰富。因此，现在是时候进行一次全球性的分裂观测调查，目的是了解哪些俯冲参数（如收敛速度、海沟迁移和曲率、下行板块的年龄和扩张历史、板块倾角和形态、地震活动、弧长、覆盖板块厚度和应力以及火山活动）似乎控制着俯冲带流场。从一个初步的调查，我们假设，3-D流占主导地位的板下，这个流场与2-D的地幔楔角流相互作用。我们将通过实验室和数值模拟研究来补充我们的主要观测地震学目标。这种正演模拟工作将用于验证我们的工作模型的预测，制定替代假设，确定任何二阶效应的流场，并探讨我们的工作模型的影响，大规模地幔动力学。从地球仪周围的许多地区的各向异性的限制和地震观测和实验室和数值模拟相结合的可用性表明，下行板块和周围地幔之间的相互作用的基本问题的解决方案是触手可及的。

What happens when a tectonic plate plunges back into the Earth?s interior at a subduction zone? Surprisingly, this simple question remains one of the fundamental unsolved problems in earth science. The flow pattern around the downgoing plate (often called the slab) is typically studied using seismic anisotropy, a property of the mantle that can be detected by recording seismic waves. In particular, measurements of shear wave splitting can be used to characterize seismic anisotropy in subduction zones. It is increasingly clear from such measurements that the simplest model of two-dimensional corner flow above the slab and entrained flow beneath the slab is likely incorrect. However, consensus on an alternative model has not been forthcoming. In this project, we will undertake a global survey of shear wave splitting observations with the goal of understanding what controls the mantle flow field in subduction zone regions. To do this, we will identify parameters that describe subduction zone dynamics that appear to exert a first-order control on shear wave splitting. Preliminary work has identified systematic variations in anisotropy both above and below the slab linked with the magnitude of trench migration velocity. This has led to the hypothesis that 3-D flow dominates beneath the slab and interacts with 2-D corner flow in the mantle wedge. In addition to a systematic evaluation of seismic anisotropy in subduction zones, we will construct laboratory and numerical models of mantle flow above and below the slab to identify diagnostic features of the flow field in shear wave splitting measurements and to explore the implications of our model for mantle dynamics. This project constitutes an interdisciplinary effort to understand and characterize the character of the mantle flow field that accompanies subduction using observations of seismic anisotropy and geodynamical modeling. With the increasing popularity of shear wave splitting as a tool for mapping mantle flow, a copious amount of data from subduction zones is now available. It is timely, therefore, to undertake a global survey of splitting observations with the goal of understanding which subduction parameters (such as convergence velocity, trench migration and curvature, age and spreading history of the downgoing plate, slab dip and morphology, seismicity, arc length, overriding plate thickness and stress, and volcanic production) appear to control the subduction zone flow field. From a preliminary survey, we hypothesize that 3-D flow dominates beneath the slab and that this flow field interacts with 2-D corner flow in the mantle wedge. We will complement our primary observational seismology goals with laboratory and numerical modeling studies. This forward modeling work will be used to validate the predictions of our working model, formulate alternative hypotheses, identify any second-order effects on the flow field, and explore the implications of our working model for larger-scale mantle dynamics. The availability of constraints on anisotropy from many regions around the globe and the combination of seismological observations and laboratory and numerical modeling suggest that a solution to the fundamental problem of interaction between downgoing slabs and the surrounding mantle is within reach.